1. Field of Invention
The present invention relates to improvements in the use of titanium alloys for the casting of dental restorations such as inlays, crowns, bridges and other dental prosthetic appliances having resistance of corrosion and oxidation under the conditions to use. According to one embodiment, the present invention relates to improved titanium alloys for the casting of dental restoration having improved receptivity and bonding strength for porcelain surfacing materials such as enamels or caps applied thereto to simulate the surface appearance of teeth.
2. Description of the Prior Art
It is well known to cast dental restorations from pure titanium and certain titanium alloys in place of noble metals such as gold and silver and alloys thereof, which are expensive and have other disadvantages such as softness, which make them susceptible to surface abrasion and distortion during use.
U.S. Pat. Nos. 4,700,769; 4,709,741; 4,768,757; 4,830,083 and 4,830,823 disclose the advantages and disadvantages of pure titanium and certain titanium alloys used for casting dental restorations.
As disclosed in U.S. Pat. No. 4,768,757, pure titanium and certain titanium alloys provide dental castings having surfaces which are soft and abrasive. A post-treatment with a nitrogen gas is required to nitride the titanium surface to increase the surface hardness and its resistance to surface abrasion.
As disclosed in U.S. Pat. Nos. 4,700,769; 4,709,741 and 4,830,083, titanium and the known titanium dental alloys are very susceptible to oxidation and require certain mold materials to prevent contamination of the dental casting and severe degradation of its physical properties, which render its unfit for its intended use.
As disclosed in U.S. Pat. No. 4,830,823 certain titanium alloys for dental castings, such as Ti-6Al-4V, are hard and brittle and require a troublesome heat after-treatment under vacuum which has precluded their practical use for dental castings. Said Patent discloses new alloys of titanium, aluminum and vanadium, specific ones of which have improved physical characteristics such as tensile strength and hardness. However the new alloys contain the same metal ingredients of Ti-6Al-4V and suffer from the same lack of resistance to oxidation.
Titanium generally is considered resistant to corrosion attack in oxidizing, neutral and inhibited reducing conditions, due to the formation of a protective surface oxide film. However, in the case of dental castings, such surface oxide films are undesirable because they tarnish the surface of the castings and interfere with the bonding properties of the surface of the castings for after-applied ceramic or porcelain surface coatings, enamels or caps applied thereover to cover or mask the metallic surface.
Titanium corrodes very rapidly in acid fluoride environments and therefore is susceptible to attack by fluoride chemicals present in toothpaste, mouthwash or liquid fluoride coatings applied to harden teeth and reduce dental carries. While mouth acidity and fluoride concentration are low, surface corrosion and surface tarnish are problems in the case of dental castings of pure titanium and the titanium alloys known for dental casting use.